Apparatus for positioning a light spot onto a character mask

ABSTRACT

AN ELECTROOPTICAL CHARACTER DISPLAY AND RECORDING DEVICE WHICH INCLUDES A DIGITAL CONTROL SYSTEM FOR ACCURATELY POSITIONING A LIGHT BEAM FROM A CATHODE-RAY TUBE ONTO A SYMBOL CONTAINED ON A MASK, SAID MASK ALSO INCLUDING A PLURALITY OF REFERENCE MARKS ALONG THE HORIZONTAL AND VERTICAL EDGES THEREOF, THE NUMBER OF MARKS TRAVERSED BY SAID BEAM ALONG ITS SCAN PATH REPRESENTING THE BEAM POSITION ON THE MASK AND MEANS FOR RECORDING SAID SYMBOL.

United States Patent COUNTING Inventor Matthew P. Tublnls Penfield, N.Y.

Appl. No. 748,367

Filed July 29, 1968 Patented June 28, 1971 Assignee Xerox CorporationRochester, N.Y.

APPARATUS FOR POSITIONING A LIGHT SPOT ONTO A CHARACTER MASK 13 Claims,6 Drawing Figs.

11.8. CI 315/10, 250/201, 250/217, 250/237, 315/85, 315/18, 328/124,340/173 Int. Cl....'. .l .Q G0lj1/20, G1 1c 1 1/26, 1101 31/26 Field ofSearch 250/217 (CRT), 201, 219 (R0), 237; 315/10, 8.5,18; IMO/324.1, 173(LM), 173 (CRT); 328/123, 124

(56] References Cited UNITED STATES PATENTS 2,855,539 10/1958 Hoover Jr,250/217X 2,929,956 3/1960 Jacobs ct a1. 250/217X 3,299,418 1/ 1967Treseder 340/324.l

' FORElGN PATENTS 1,192,975 4/1959 France 328/124 Primary Examiner-JamesW Lawrence Assisiant Examiner-T. N. Grigsby Anomeys- Paul M. Enlow,Ronald Zibelii, James J. Ralabate and Norman E. Schrader CONTROL 8.HORIZONTAL DE F L ECTION CIRCUIT GENERATOR VERTICAL DEFLECTION GENERATORPmEminJuuzwl 3,588,584

SHEET 1 OF 4 CONTROL a HORIZONTAL v COUNTING DEFL ECTION 204 cmcunGENERATOR VERTICAL 20a DEFLECTION 2 GENERATOR 2 FIG.

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ml, DETECTOR 326 24 2/2 340' H BINARY D CRT H RESET 'l/ 220 DEF'LECT.

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r300 CHARACTER R JC DECODER 342 .234 .336 T L MPAR N TUNBLANK cm: cmcun'CONTROL 'smo 356' 354 VERTICAL 5 DEFLECTION 330 3 3 COUNTERPAIENIEnJuueslsn 35 ,5 21.

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I I r0 IVERTICAL IDEFLECT YOKE 202 VERTICAL DEF ECTION GENERATOR SAME ASABOV E) PATENTEU JUH28 297i SHEET 0F 4 APPARATUS FOR POSITIONING A LIGHTSPOT ONTO A CHARACTER MASK BACKGROUND OF THE INVENTION An electroopticalcharacter display system, as the term is used herein, refers to a devicefor forming a beam of light in the shape of a character or forpositioning the light beam onto a mask containing alphanumericalinformation and including electrically controlled beam deflection meansfor selectively positioning the beam of light at a predeterminedposition on the mask, a viewing screen or on a photosensitive member forrecordation thereon. The most common example of the term is acathode-ray tube (CRT), but the term, as well as the present invention,also applies to related devices such as those employing mirrorgalvanometers or piezoelectrically or magnetorestrictively drivenmirrors for beam deflection or employing electrooptical effects inelectrically active crystals. The characters referred to may be simplyspots of light which are displayed at preselected positions, but morecommonly will comprise a set of numerals, letters and symbols so thatthe display device may write words, numbers or some mixture thereof.Such devices are particularly useful for the writing and instantaneousdisplay of information from high speed digital computers. Withimprovements in cathode-ray tube technology and with an ever increasingneed for greater speed in recording output information from computersand high speed telecommunications circuits, recording devices of theindicated kind are beginning to displace electromechanical printers suchas teletypewriters or high speed line-at-a-time computer printers-Thisis particularly so where the information is to be recorded in aminiaturized format, for which electromechanical printers are quiteunsuited. However, electrooptical display devices have suffered from aserious defect as compared to electromechanical devices. The lattercharacteristically prints characters at accurately spaced intervalsalong a straight line, but this precise character alignment andpositioning has been difficult to achieve with electrooptical printers.For example, if a perfectly linear relation is established between anangle of deflection and a beam voltage, the displacement of the beam ona flat display or recording surface will vary as the tangent of thecontrol voltage rather than the voltage itself. Even this situation isnot achieved in a practice because the beam deflection angle itself isnot strictly linearly related to the control voltage. Pincushiondistortion also effects alignment as the horizontal and vertical traceson the CRT face are forced in along the axis and out along the diagonal.Finally, the high speed electronic circuits which are required togenerate the control voltages corresponding to a great many characterpositions tend to vary with time in an unpredictable fashion, as may thedeflection system itself. A partial solution to this problem may be hadby employing nonlinear compensating circuits together with other highlystabilized electronic circuits. However, the required circuits areexpensive and difficult to adjust and still fail to provide the ultimatein character positioning accuracy.

SUMMARY OF THE INVENTION The present invention relates to electroopticaldisplay devices and, more particularly, to optical feedback means forproviding a fast and accurate method for eliminating positional errorsin the output display.

The present invention includes a mask, having. symbols printedthereupon, interposed between the output of a cathode-ray tube and aphotodetector. The mask also includes a number of marks positioned alongits horizontal and vertical edges which eliminates the linearity problemcaused by pincushion distortion. The light beam from the cathode-raytube scans the mask along the edges, the photodetector sensing thenumber of marks passed along the scan path. Theelectrical output of thedetector is fed back to a digital control system which compares theposition of the light beam on the mask with a desired position,represented by an encoded input signal. The position of the beam isadjusted by the control system until the beam strikes the desired symbolon the mask.

It is, accordingly, the principal object of the invention to provide anelectrooptical display and recording means for precise characterpositioning and recording.

It is a further object of the invention to provide an electroopticaldisplay means for fast and accurate character positioning byelectrooptical a novel digital control system to position a cathode-raytube beam.

It is still a further object of the invention to provide anelectrooptical display means for precise character positioning whichutilizes a mask with indexing marks along the horizontal and verticaledges thereof, the mask also compensating for pincushion distortion.

Other objects and features of the present invention will become apparentfrom the following detailed description when taken in conjunction withthe drawings in which:

FIG. I is a block diagram of a preferred embodiment of the invention;

FIG. 2 shows the novel digital control system utilized in the invention;

FIG. 3 is a schematic diagram of the horizontal and vertical deflectionsignal generators; and

FIGS. 4(a) and 4(b) illustrate the optical mask system utilized in theinvention.

FIG. 5 illustrates another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 there is shown acathode-ray tube 200 including deflection means 202 which is focusedthrough a mask 204 having symbols, such as alphanumerical information,thereon and optical system 206 onto photodetector 208. Deflection means202 would ordinarily be a magnetic yoke or a set of internalelectrostatic plates. For the purposes of the present invention,cathode-ray tube 200 and deflection means 202 can be replaced by anysuitable fixed light source operating in con junction with anelectrically controllable means for deflecting a beam of light from thelight source. The illuminated character on mask 204 is recorded byfocusing the light passing through optical system 206 on a viewingscreen or on any form of photosensitive material which can convert thelight beam into a permanent image, a xerographic drum being illustrativeof such a photosensitive member. Since recorders employing xerographictechniques are in wide spread use and have been extensively described inthe patent and technical literature, no further description of thephotosensitive means or of the auxiliary equipment, such as charging,developing and paper handling equipment for use therewith will be given.An example of a recording technique is disclosed in copending U.S. Pat.application Ser. No. 583,542, filed Oct. 3, 1966. The described elementsof FIG. 1 are typical of electro-optical display devices. The deflectionmeans 202 will initially incrementally displace the light beam or spotalong the mask 204 in a horizontal direction. After the beam ispositioned in its proper location, the deflection means willincrementally displace the light beam to the proper vertical position.The light output from cathode-ray tube 200 passing through mask 204 isfocused by optical system 206 onto the photodetector 208. The electricaloutput of photodetector 208 is fed to digital control circuit 214, theoutput thereof controlling horizontal deflection generator 218, verticaldeflection generator 222 and the cathode-ray tube grid via lead 224 inorder to unblank the beam in a manner which will be more fully explainedin connection with the subsequent FIGS. Mask 204 has a plurality ofslits along its horizontal and vertical edges and alphanumeric materialthereon, the slits being utilized to position the beam to the desiredposition according to the data input information and also to correct forpincushion distortion. In an alternate embodiment, transparent materialmay be used in place of the slits for indexing purposes. In the mode ofoperation of the invention generally applicable to FIG. I, the datainput to the control system 214 initially causes a signal to betransmitted to horizontal deflection generator 218 and vertic'aldeflection generator 222 via leads 216 and 220, respectively. The outputof the deflection generators 218 and 222 energizes the deflection coil202, the coil deflecting the beam to a predetermined position. Utilizingapparatus described in greater detail in FIG. 3, the horizontaldeflection generator is energized initially, causing the beam to movealong the upper horizontal edge of mask 204, the beam striking the slitspositioned along its scan length. The number of slits the beam passes asit traverses its scan path is sensed by photodetector 208, thisinformation being fed back to the control circuit 214 via lead 212. Thecontrol means 214 continues to generate an output signal to horizontaldeflection generator 218 until the desired position on mask 204 isattained. The beam is then returned to the initial position and asimilar procedure is then followed with reference to the verticaldeflection generator 222. The beam is caused to move along the verticaledge of the mask striking the slits positioned along its scan length.The number of slits struck by the beam is sensed by the photodetectorand fed to control circuit 214 via lead 212. When the proper verticalposition is reached, the horizontal deflection voltage is reapplied,positioning the beam at the proper point on the mask. The controlcircuit 214 also controls the unblanking of cathode-ray tube 200 vialead 224 and operates essentially to unblank the beam during the timewhen the beam scans the horizontal and vertical edges of the mask andwhen the beam reaches its proper position on the mask.

Referring now to FIG. 2, there is shown in greater detail the controlcircuitry 214 described in FIG. 1.

Upon receipt of the input data at character decoder 300, an output pulsetherefrom is directed to AND gate 302 which allows clock 304 to drivebinary counter 306. The output of binary counter 306 drivesdigital-analog converter 308 to provide an analog voltage for horizontaldeflection generator 218 via lead 216. Switch 310 is positioned in itslower position allowing the signal from the digital analog converter tobe applied to the deflection circuit. The voltage steps necessary tosweep the beam would be finely divided requiring approximately ten stepsto drive the beam between the slits in line with each character. As thespot passes each slit in the horizontal direction, photodetector 210detects the light at each slit and produces a count on line 212. Thissignal drives the horizontal deflection counter 314 when switch 322 isin the position shown. The horizontal position memory 316 is preset tosome number corresponding to the next character position to be printed.A continual comparison is made between the horizontal deflection counter314 and the horizontal position memory 316 so that when the numbersstored in each are identical, a negative pulse corresponding to thelogic level is generated by the comparison circuit 318 and applied togate 302, the gate now being closed to prevent any further pulses fromentering the counter 306. At this time, a pulse also generated bycomparison circuit 318, energizes switch control means 320. The switchcontrol enables contact 3100 of switch 310 to be connected to lead 420,the zero level voltage source 312 being applied to horizontal deflectiongenerator 218 (FIG. 3) via lead 420 which operates to return the beam toits initial resting position. Switch control 320 operates simultaneouslyupon switch 322 and places the switch at contact 322b. Pulses from clock304 are applied to binary counter 324 via AND gate 326. The output ofthe binary counter 324 is connected to digital-analog converter 328, theoutput thereof driving the vertical deflection generator of thecathode-ray tube 200 via lead 220 and causing the beam to be drivenalong the vertical edge of mask 204. Photodetector 208 senses the numberof slits exposed along the beam scan path and provides pulsesrepresenting this number to the vertical deflection counter 330 via lead212 and contact 322b. Comparator circuit 334 compares this number withthe number contained in the data input and decoded by character decoder300. When these two numbers are equal, a negative output pulse (logical0) is generated by the comparator circuit 334, closing gate 326, therebyinhibiting any further clock pulses to pass to binary counter 324. Theoutput of comparator circuit 334 is also applied to unblank controlmeans 336, the output thereof feeding switch control means 320, causingthe output of counter 306 to be applied to lead 216 via digitalanalogconverter 308 and contact 310b and allowing the voltage to be applied tothe horizontal deflection circuit, thereby moving the beam to a positionwhich exposes the proper character on mask 204. The output of unblankingcontrol means 336 is also applied to the grid circuit of the cathode-raytube 200 via lead 224 which unblanks the beam when it is properlypositioned. The cathode spot or beam is rastered about this position insome manner and the counters 306 and 324 are then reset to zero byapplying the output pulse from comparator circuit 334 to reset means 338and 340 via delay means 342. The output of the delay means 342 is alsoapplied to gate 344 which advances horizontal position memory 316 onecount for printing the next character.

Referring now to FIG. 3, there is shown a schematic diagram of ahorizontal deflection generator used in the present invention.

The basic operation of the generator shown in FIG. 3 is as follows: Thevoltage output generated by counter 306 (FIG. 2) and transmitted byinput line 216, controls the input to transistor T1. When T1 isconducting, capacitor 404 charges towards Ecc, resulting in a rampoutput from the Darlington pair comprising transistors T2 and T3. Theoutput of transistor T3 is applied to a voltage attenuator made up ofresistors 406, 408, 410 and 412 which drives linear voltage amplifier414. The output of amplifier 414 drives a current amplifier 416 todeflect the cathode-ray tube beam along a horizontal axis. As explainedhereinbefore, as the beam is detected passing a slit, a pulse istransmitted to horizontal deflection counter 314 (FIG. 2). When apredetermined number of pulses has been detected, source 312 applies aground on lead 420 via contact 310a causing transistor T1 to be reversedbias. The transistor no longer conducts and the charge on capacitor 404is retained at the proper horizontal deflection voltage.

At the time when lead 420 applies the ground potential to the base oftransistor T1, a negative pulse from comparison circuit 318 is appliedto gate 350 via lead 352. Gate 350, initially at a zero voltage,corresponding to a logical l, is disabled and the negative pulsegenerated at its output drives transistor T4 into conduction, forwardbiasing diode 424, limiting the output of amplifier 414 to its initialvalue. The cathode-ray tube beam is therefore returned to its restposition and the vertical deflection cycle now is initiated as describedhereinbefore. When the desired vertical position has been reached, apulse generated by comparator 334 is transmitted to gate 350 via lead353, driving T4 to its initial condition allowing the voltage oncapacitor 404 to position the beam to the correct horizontal position atthe same instant that the correct vertical voltage is attained at theoutput of vertical deflection generator 222. A modulating signal can beapplied to voltage amplifier 414 for rastering the spot about thedesired position. The output of transistor T5 can be used by focusingcircuitry not disclosed herein for correcting the spot size on thecathoderay tube surface. Transistor T6 is a shunting control switch canbe used to apply a correcting voltage to the amplifier in case the driftof the spot exceeds the desired level. Transistor T7 is a capacitordumping circuit which is utilized to forward bias diode 426, providing adischarge path for capacitor 404 after the beam is positioned properly.Transistor T7 is driven to conduction by the negative pulse generated bycomparator 334 and applied to lead 356 via delay circuit 354.

Referring now to FIG. 4a, mask 204 includes a plurality of slits orother indexing marks 500 extending along the horizontal edge and aplurality of slits 502 extending along the vertical edge of the mask.The light beam is initially at rest at location 504 and when voltage isapplied to the horizontal deflection generator 218, the beam isinitially deflected along the horizontal edge in the direction of arrow506. When the beam reaches a slit corresponding to the desired symbol,the beam is reset to position 504 by the apparatus hereinbeforedisclosed in reference to FIGS. 2 and 3. Subsequently, the beam isdeflected downwards in the direction of arrow 508. Assuming that thevertical position has been reached, and that it is desired to positionthe beam at point 510, the reset mode of the horizontal deflectiongenerator is removed and both the horizontal and vertical deflectionvoltages are applied simultaneously to deflection coils 202 illustratedin FIG. I. The beam will then be positioned at 510. As is obvious fromthe particular character mask 204 used, the horizontal deflectiondictates only where the selected character will be located in the finaloptical display, while the vertical deflection actually determines whatcharacter will be displayed.

Referring now to FIG. 4b, there is shown in greater detail a portion ofthe mask 204 shown at location 512. Illustratively, the slit width C isapproximately 0.004 inches, the character width d is approximately 0.017inches and the approximate spacing between characters e is 0.005 inches.The system disclosed has a positional accuracy of about one part inthree hundred to the middle of the character for a spot size diameter ofbetween 2 and 3 mils.

FIG. 5 illustrates an arrangement wherein a plurality of photosensitivemembers 600, 602, 604, and 606 are utilized as detectors in place of asingle photodetector. The photosensitive members are positioned on mask204 over corresponding indexing marks. As the light beam generated bycathode-ray tube 200 initially scans the portion of the horizontal edgeof mask 204 illustrated, each of the photosensitive members 600 and 602generate an electrical pulse as the beam impinges thereon. The outputpulses are transmitted to the horizontal deflection counter 314 (FIG. 2)via OR gate 608, contact 322a of switch 322' and lead 610. After thehorizontal scan is completed, the beam returns to an initial position504. The beam is now deflected along the vertical edge of mask 204,photosensitive members 604 and 606 each generating a pulse as the beamimpinges thereon. The outputs of members 604 and 606 are coupled tovertical deflection counter 330 (FIG. 2) via OR gate 608', contact 322kof switch 322 and lead 612. The double throw switch 322' replaces switch322 utilized in the embodiment shown in FIG. 2. Switch control 320 nowoperates to change the contact positions shown in FIG. 5, after thehorizontal scan is completed, i.e., contact 322a would be disconnectedfrom lead 610 and contact 322b is connected to lead 612.

The output leads of the photosensitive members along each edgealternately maybe soldered together and then connected to the respectivecontacts of switch 322', eliminating the necessity of OR gates 608 and608'.

While the present invention has been described with reference to itspreferred embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from its essential teachings.

lclaim:

I. Apparatus for controlling the position of a light spot includingmeans for deflecting said light spot along horizontal and verticaldirections in response to signals applied thereto, wherein theimprovement comprises:

a mask positioned to receive said light spot, said mask having aplurality of discrete, spaced index marks along its horizontal andvertical edges to indicate the horizontal and vertical position of saidlight spot, said index marks passing light incident thereon;

photodetector means positioned with respect to said marks so that lightpassed by each of said index marks produces an output pulse therefrom;and

control means responsive to output pulses from said photodetector meansfor comparing the position of said light spot on said mask as determinedby said output pulses with a predetermined light spot position, saiddeflection means being responsive to the output of said control meansto. adjust the position of said light spot to said predeterminedposition.

2. The apparatus as defined in claim 1 wherein said mask includesalphanumerical symbols thereon, said symbols passing light incidentthereon.

3. The apparatus as defined in claim 2 further including photosensitiverecording means for recording light passed by said mask, the lightpassed by said mask being in the shape of the symbol located at saidpredetermined position.

4. The apparatus as defined in claim 3 wherein said control meansfurther includes means for resetting said light spot to an initialposition after said light spot has been deflected to a predeterminedposition along one of said mask edges and means for redeflecting saidlight spot to said predetermined position after said light spot isdeflected to a predetermined position along the other mask edge.

5. The apparatus of claim 1 in which the source of said light spotcomprises a cathode-ray tube.

6. The apparatus of claim 5 further including means connected to saidcontrol means for unblanking said cathode-ray tube during the time whensaid light spot is deflected along said horizontal and verticaldirections and when said light spot reaches said predetermined positionon said mask.

7. The apparatus as defined in claim 6 further including means to resetsaid control system after said predetermined position is attained. I I

8. Apparatus for optically displaying an alphanumerical symbol on a maskincluding a cathode-ray tube, means to form a light spot on the face ofsaid cathode-ray tube, a horizontal deflection drive circuit forcontrolling the horizontal position of said light spot, and a verticaldeflective drive circuit for controlling the vertical position of saidlight spot, wherein the improvement comprises:

a mask positioned near the face of said cathode-ray tube and having aplurality of discrete, light transmitting spaced index marks along itshorizontal and vertical edges to indicate the horizontal and verticalposition of said light spot and light transmitting alphanumericalsymbols thereon;

photodetector means positioned with respect to said mask so that lighttransmitted by each of said index marks produce an output pulsetherefrom;

control means responsive to the output pulses of said photodetectormeans for comparing the actual position of said light spot on said maskas determined by said output pulses with a predetermined light spotposition, said control means generating signals representing thedifference between said actual and predetermined light spot positions,said horizontal and vertical drive circuits being responsive to saiddifference signals whereby said light spot is positioned to saidpredetermined position; and

means for unblanking said cathode-ray tube during the times when lightis transmitted by said horizontal and vertical edge index marks and whensaid light spot is at said predetermined position.

9. The apparatus as defined in claim 8 wherein said light spot exposes aselected alphanumerical symbol on said mask when said light spot isdeflected to said predetermined position and further includingphotosensitive recording means for recording the light transmitted bysaid selected symbol.

10. The device as defined in claim 9 wherein said control meanscomprises first and second position counters coupled to the output ofsaid photodetector means, and first and second comparator circuits, theoutput of said first counter being compared with a predetermined firstposition in said first comparator and the output of said second counterbeing compared with a predetermined second position in said secondcomparator, the output of said first and second comparators controllingsaid drive circuits.

11. Apparatus for optically displaying an alphanumerical symbol on amask including a cathode-ray tube, means to form a light spot on theface of said cathode-ray tube, a horizontal deflection drive circuit forcontrolling the horizontal position of said light spot, and a verticaldeflection drive circuit for controlling the vertical position of saidlight spot, wherein the improvement comprises:

a mask positioned near the face of said cathode-ray tube and having aplurality of discrete, light transmitting slits along its horizontal andvertical edges to indicate the horizontal and vertical position of saidlight spot and light transmitting alphanumerical symbols thereon;

photodetector means positioned with respect to said mask so that lighttransmitted by each of said slits produce an output pulse therefrom;

control means responsive to the output pulses of said photodetectormeans for comparing the actual position of said light spot on said maskas determined by said output pulses with a predetermined light spotposition, said control means generating signals representing thedifference between said actual and predetermined light spot positions,said horizontal and vertical drive circuits being responsive to saiddifference signals whereby said light spot is positioned to saidpredetermined position; and

means for unblanking said cathode-ray tube during the times when lightis transmitted by said horizontal and vertical edge slits and when saidlight spot is at said predetermined position.

12. Apparatus for optically displaying an alphanumerical symbol on amask including a cathode-ray tube, means to form a light spot on theface of said cathode-ray tube, a horizontal deflection drive circuit forcontrolling the horizontal position of said light spot, and a verticaldeflection drive circuit for controlling the vertical position of saidlight spot, wherein the improvement comprises:

a mask positioned near the face of said cathode-ray tube and having aplurality of discrete, light transmitting spaced index marks along itshorizontal and vertical edges to indicate the horizontal and verticalposition of said light spot and light transmitting alphanumericalsymbols thereon;

photodetector means positioned with respect to said mask so that lighttransmitted by each of said index marks produce an output pulsetherefrom;

control means responsive to the output pulses of said photodetectormeans for comparing the actual position of said light spot on said maskas determined by said output pulses with a predetermined light spotposition, said control means generating signals representing thedifference between said actual and predetermined light spot positions,said horizontal and vertical drive circuits being responsive to saiddifference signals whereby said light spot is positioned to saidpredetermined position, said control means comprising a horizontaldeflection counter for counting the pulses generated by saidphotodetecting means as said light spot is driven along the horizontaledge of said mask, said pulses representing the actual horizontalposition of said light spot;

first comparator circuit connected to said horizontal deflectioncounter;

horizontal position memory connected to said first comparator;

vertical deflection counter for counting the pulses generated by saidphotodetecting means as said light spot is driven along the verticaledge of said mask, said pulses representing the vertical position ofsaid light spot;

second comparator circuit connected to said vertical deflection counter;

character decoder, the input of which is connected to a source of inputdata said character decoder converting said input data into horizontaland vertical position information the output of said character decoderbeing coupled to said second comparator circuit;

a first binary counter coupled to the output of said first comparator;

a second binary counter coupled to the output of said second comparatorcircuit;

means for coupling the output of said first binary counter to saidhorizontal deflection drive circuit;

means for coupling the output of said second binary counter to said vertical deflection drive circuit; means for switching the output of saidphotodetector means from energizing said horizontal deflection counterto said vertical deflection counter after a predetermined horizontalposition is attained; and means for unblanking said cathode-ray tubeduring the times when light is transmitted by said horizontal andvertical edge index marks and when said light spot is at saidpredetermined position. 13. The apparatus as defined in claim 12 furtherincluding means for resetting said first and second binary countersafter said light spot is positioned at said predetennined position.

